As for an electrophotographic photoreceptor, those containing each an inorganic photoconductor such as selenium, zinc oxide, cadmium sulfide and silicon as the principal component have heretofore been widely known. However, their characteristics such as the thermal stability and durability thereof have not always been satisfactory and they have also had the problems of manufacture and handling.
On the other hand, the photoreceptors comprising each a photoreceptive layer containing an organic photoconductive compound as the principal component thereof are relatively easier for manufacture, inexpensive in cost, easier for handling and, generally, excellent in thermal stability as compared to a selenium photoreceptor. As for the organic photoconductive compounds, a poly-N-vinylcarbazole has been well-known. There have already been put to practical use the photoreceptors comprising each a photoreceptive layer containing an electric-charge transfer complex produced of the above-mentioned poly-N-vinylcarbazole and a Lewis acid such as 2,4,7-trinitro-9-fluorenone as the principal component thereof.
Further, on the other hand, there have also been known a photoreceptor comprising a laminated or single layered function-separated type photoreceptive layer in which the carrier generating function and carrier transporting function of the photoconductor thereof can be performed by separate materials, respectively. For example, there have already been put to practical use the photoreceptors comprising each a photoreceptive layer comprising a carrier generating layer formed of a thin amorphous selenium layer and a carrier transporting layer containing a poly-N-vinylcarbazole as the principal component thereof.
However, the above-mentioned poly-N-vinylcarbazole lacks a flexibility and the coated layer thereof is solid and fragile and is liable to be cracked or peeled off. Every photoreceptor applied therewith is, therefore, deteriorated in durability. When the above-mentioned defects are improved by adding a plasticizer, the residual potentials are increased in an electrophotographic process and are accumulated as in making repetition use so as to produce fogs increasingly, so that a copied image is spoiled.
A low-molecular organic photoconductive compound has generally no coated-layer forming function. Therefore, when the compound is used with a suitable binder in combination and the kinds and composite proportions of the binders are suitably selected, the compound is preferable from the viewpoint that the physical properties of the coated layer or the photoreceptive characteristics can each be controlled to some extent. However, there are some limitation to the kinds of the organic photoconductive compounds each having a relatively high compatibility with the binders. Actually, there are limited to the kinds of the binders applicable to form the photoreceptive layers of an electrophotographic photoreceptor.
For example, 2,5-bis(p-diethylaminophenyl)-1,3,4-oxadiazole described in U.S. Pat. No. 3,189,447 is low in compatibility with the binders commonly applicable as a material of the photoreceptive layers of an electrophotographic photoreceptor, such as a polyester and a polycarbonate. In other words, when a photoreceptive layer is formed by mixing them up in a proportion required for controlling the electrophotographic characteristics, the crystals of oxadiazole are deposited at a temperature of not lower than 50.degree. C., so as to have a defect that the electrophotographic characteristics such as electric-charge coercive force and photoreceptivity deteriorate.
In the meanwhile, each of the diaryl alkane derivatives described in U.S. Pat. No. 3,820,989 has few problems of the compatibility with a binder. However, it is low in stability against light. When it is applied to a repetition-transfer type electrophotographic photoreceptor to which an electric-charge.multidot.an exposure are applied repeatedly, it has a defect that the photoreceptivity of the photoreceptive layer is gradually lowered.
In U.S. Pat. No. 3,274,000 and Japanese Patent Examined Publication No. 47-36428/1972, the different types of phenothiazine derivatives are described, respectively. However, every one of them has the defect that the photoreceptivities thereof are low and the stabilities thereof are also low in repetition use.
The stilbene compounds described in Japanese Patent Publication Open to Public Inspection (hereinafter referred to as JP OPI Publication) Nos. 58-65440/1983, 58-190953/1983 and 63-149652/1988 are each relatively excellent in electric-charge coercive force and photoreceptivity. However, they are still not satisfactory in durability in repetition use.
In the meanwhile, the photoreceptors capable of solving the above-mentioned defects include those applied with a distilbene compound as the carrier transporting material thereof, such as those described in JP OPI Publication Nos. 60-175052/1985, 60-174749/1985, 62-120346/1987, 64-32265/1989, 1-106069/1989, 1-93746/1989 and 1-274154/1989. However, they can still not be satisfactory in durability when they are incorporated into a copier or a printer and are used repeatedly.
Particularly, there have been the following for problems.
1) In a high-speed copier having a high linear velocity, a cycle of electric-charging.multidot.exposure.multidot.electric neutralization is shortened. Therefore, the residual potential is seriously raised in repetition copying operations;
2) When making use of a copier for repeating copying operations, a small white spotted image defect (that is so-called a white-dot) is produced in solid-black image areas;
3) When making use of a reversal development type printer, there found the defects that an exposure potential (VL) is raised at a low temperature and an electric-charge potential (VH) is lowered in repetition use; and
4) A small black-spotted image defect (that is so-called a black-dot) is liable to be produced in white image areas.
As described above, there has not been found any carrier transporting material having the characteristics which can be practical and satisfactory for providing an electrophotographic photoreceptor.